The operation of a 90.degree. V6 four stroke cycle internal combustion piston engine having offset crankpins on each throw to provide even firing intervals is such that movement of the piston-crank mechanism causes a mechanical imbalance comprising an eccentric primary rotating unbalance couple turning in the direction of crankshaft rotation and an eccentric secondary rotating unbalance couple turning opposite the direction of crankshaft rotation. The rotating forces of these unbalanced couples are cyclically variable, having generally egg-shaped or elliptical configurations, when graphically presented, with their smaller moment axes lying in a longitudinal plane (often extending vertically) spaced equally between the cylinder banks and their larger moment axes extending (usually horizontally) at right angles to the longitudinal plane between the cylinder banks.
To fully balance these rotating couples would require four sets of longitudinally spaced balance weights including two pairs oppositely rotated at crankshaft speed and two pairs oppositely rotated at twice crankshaft speed. A substantial portion of the primary unbalance can be easily offset by the application of counterweights to the engine crankshaft in accordance with common practice. However, offsetting the remaining primary and secondary unbalance requires the addition of extra balancing mechanism, adding additional complexity and cost to the engine structure. Thus, in many applications the residual primary and secondary unbalance is not offset but, instead, is absorbed in resilient engine mounting systems which reduce the transmission of engine vibrations to the vehicle frame, or other structure in which the engine may be mounted.